Vehicle speed may be controlled through various mechanisms, including brakes and retarders. Brakes and retarders may be used to slow a vehicle or prevent acceleration, such as during descents. Retarders may be used instead of brakes to minimize the wear and localized heat caused by brake applications, particularly if a descent is long, steep, or undertaken while the vehicle is fully loaded.
In those applications where it is advantageous for a vehicle to use a retarder instead of brakes, it may be desirable to make the application of the retarder as easy and automatic as possible to encourage its usage where appropriate. It may also be desirable to reduce the number of cycles the retarder experiences to reduce the wear associated with cycling the retarder and any discontinuities in vehicle acceleration such cycling may cause.